Messages in this thread | | | From | Vincent Guittot <> | Date | Thu, 12 Mar 2020 13:27:53 +0100 | Subject | Re: [PATCH V2] sched: fair: Use the earliest break even |
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On Thu, 12 Mar 2020 at 11:04, Daniel Lezcano <daniel.lezcano@linaro.org> wrote: > > On 12/03/2020 09:36, Vincent Guittot wrote: > > Hi Daniel, > > > > On Wed, 11 Mar 2020 at 21:28, Daniel Lezcano <daniel.lezcano@linaro.org> wrote: > >> > >> In the idle CPU selection process occuring in the slow path via the > >> find_idlest_group_cpu() function, we pick up in priority an idle CPU > >> with the shallowest idle state otherwise we fall back to the least > >> loaded CPU. > > > > The idea makes sense but this path is only used by fork and exec so > > I'm not sure about the real impact > > I agree the fork / exec path is called much less often than the wake > path but it makes more sense for the decision. > > >> In order to be more energy efficient but without impacting the > >> performances, let's use another criteria: the break even deadline. > >> > >> At idle time, when we store the idle state the CPU is entering in, we > >> compute the next deadline where the CPU could be woken up without > >> spending more energy to sleep. > >> > >> At the selection process, we use the shallowest CPU but in addition we > >> choose the one with the minimal break even deadline instead of relying > >> on the idle_timestamp. When the CPU is idle, the timestamp has less > >> meaning because the CPU could have wake up and sleep again several times > >> without exiting the idle loop. In this case the break even deadline is > >> more relevant as it increases the probability of choosing a CPU which > >> reached its break even. > >> > >> Tested on: > >> - a synquacer 24 cores, 6 sched domains > >> - a hikey960 HMP 8 cores, 2 sched domains, with the EAS and energy probe > >> > >> sched/perf and messaging does not show a performance regression. Ran > >> 50 times schbench, adrestia and forkbench. > >> > >> The tools described at https://lwn.net/Articles/724935/ > >> > >> -------------------------------------------------------------- > >> | Synquacer | With break even | Without break even | > >> -------------------------------------------------------------- > >> | schbench *99.0th | 14844.8 | 15017.6 | > >> | adrestia / periodic | 57.95 | 57 | > >> | adrestia / single | 49.3 | 55.4 | > >> -------------------------------------------------------------- > > > > Have you got some figures or cpuidle statistics for the syncquacer ? > > No, and we just noticed the syncquacer has a bug in the firmware and > does not actually go to the idle states. > > > >> | Hikey960 | With break even | Without break even | > >> -------------------------------------------------------------- > >> | schbench *99.0th | 56140.8 | 56256 | > >> | schbench energy | 153.575 | 152.676 | > >> | adrestia / periodic | 4.98 | 5.2 | > >> | adrestia / single | 9.02 | 9.12 | > >> | adrestia energy | 1.18 | 1.233 | > >> | forkbench | 7.971 | 8.05 | > >> | forkbench energy | 9.37 | 9.42 | > >> -------------------------------------------------------------- > >> > >> Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org> > >> --- > >> kernel/sched/fair.c | 18 ++++++++++++++++-- > >> kernel/sched/idle.c | 8 +++++++- > >> kernel/sched/sched.h | 20 ++++++++++++++++++++ > >> 3 files changed, 43 insertions(+), 3 deletions(-) > >> > >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > >> index 4b5d5e5e701e..8bd6ea148db7 100644 > >> --- a/kernel/sched/fair.c > >> +++ b/kernel/sched/fair.c > >> @@ -5793,6 +5793,7 @@ find_idlest_group_cpu(struct sched_group *group, struct task_struct *p, int this > >> { > >> unsigned long load, min_load = ULONG_MAX; > >> unsigned int min_exit_latency = UINT_MAX; > >> + s64 min_break_even = S64_MAX; > >> u64 latest_idle_timestamp = 0; > >> int least_loaded_cpu = this_cpu; > >> int shallowest_idle_cpu = -1; > >> @@ -5810,6 +5811,8 @@ find_idlest_group_cpu(struct sched_group *group, struct task_struct *p, int this > >> if (available_idle_cpu(i)) { > >> struct rq *rq = cpu_rq(i); > >> struct cpuidle_state *idle = idle_get_state(rq); > >> + s64 break_even = idle_get_break_even(rq); > >> + > >> if (idle && idle->exit_latency < min_exit_latency) { > >> /* > >> * We give priority to a CPU whose idle state > >> @@ -5817,10 +5820,21 @@ find_idlest_group_cpu(struct sched_group *group, struct task_struct *p, int this > >> * of any idle timestamp. > >> */ > >> min_exit_latency = idle->exit_latency; > >> + min_break_even = break_even; > >> latest_idle_timestamp = rq->idle_stamp; > >> shallowest_idle_cpu = i; > >> - } else if ((!idle || idle->exit_latency == min_exit_latency) && > >> - rq->idle_stamp > latest_idle_timestamp) { > >> + } else if ((idle && idle->exit_latency == min_exit_latency) && > >> + break_even < min_break_even) { > >> + /* > >> + * We give priority to the shallowest > >> + * idle states with the minimal break > >> + * even deadline to decrease the > >> + * probability to choose a CPU which > >> + * did not reach its break even yet > >> + */ > >> + min_break_even = break_even; > >> + shallowest_idle_cpu = i; > >> + } else if (!idle && rq->idle_stamp > latest_idle_timestamp) { > >> /* > >> * If equal or no active idle state, then > >> * the most recently idled CPU might have > >> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c > >> index b743bf38f08f..3342e7bae072 100644 > >> --- a/kernel/sched/idle.c > >> +++ b/kernel/sched/idle.c > >> @@ -19,7 +19,13 @@ extern char __cpuidle_text_start[], __cpuidle_text_end[]; > >> */ > >> void sched_idle_set_state(struct cpuidle_state *idle_state) > >> { > >> - idle_set_state(this_rq(), idle_state); > >> + struct rq *rq = this_rq(); > >> + > >> + idle_set_state(rq, idle_state); > > > > Shouldn't the state be set after setting break even otherwise you will > > have a time window with an idle_state != null but the break_even still > > set to the previous value > > IIUC we are protected in this section. Otherwise the routine above would > be also wrong [if (idle && idle->exit_latency)], no?
no there are not the same because it uses the idle pointer to read exit_latency so we are sure to use exit_latency related to the idle pointer.
In your case it checks idle is not null but then it uses rq to read break_even but it might not have been already updated
> > >> + > >> + if (idle_state) > >> + idle_set_break_even(rq, ktime_get_ns() + > > > > What worries me a bit is that it adds one ktime_get call each time a > > cpu enters idle > > Right, we can improve this in the future by folding the local_clock() in > cpuidle when entering idle with this ktime_get.
Using local_clock() would be more latency friendly
> > >> + idle_state->exit_latency_ns); > >> } > > [ ... ] > > > -- > <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs > > Follow Linaro: <http://www.facebook.com/pages/Linaro> Facebook | > <http://twitter.com/#!/linaroorg> Twitter | > <http://www.linaro.org/linaro-blog/> Blog >
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